This invention relates in general to electro-optic modulators and, more specifically for arrangements for compensating for strain and temperature changes in such a modulator.
Over the years, systems have come into use which use electrical signals passing through wires from input means to sense information and return the information to a distant location for use. These systems are much lighter in weight, occupy little space and provide redundant wiring paths to protect against loss of contact if one wire is damaged. Unfortunately, these systems are subject to short circuits or other damage to the wires, electromagnetic interference (EMI) from nearby wiring or electrical devices and are potentially subject to destruction by electromagnetic pulses (EMP) from nuclear blasts or other sources. There is a particular need to overcome these problems in military aircraft, missiles and ships and in numerically controlled machine tools and robotics where EMI and EMP pose serious problems.
Recently, considerable interest has developed in using optical fiber systems for passing information rapidly and accurately from a remote sensor over long distances. Optical fiber systems have many advantages over the wired electrical circuits EMI and EMP, short circuit potential and are lighter in weight which is very important for aerospace applications. Typical fiber optic control systems are disclosed by Sichling in U.S. Pat. No. 4,246,478 and Blackington in U.S. Pat. No. 4,313,226.
Many specialized devices have been used in optical fiber systems for transducing electrical or mechanical position or quantity signals into a proportional light signal suitable for transmission through a fiber. Typical of these are the systems disclosed by Walker in U.S. Pat. No. 4,454,418 and Lockett et al. in U.S. Pat. No. 4,479,264. In many cases it is necessary to transmit light from a friendly environment (e.g., low temperature with no electro magnetic interference such as an aircraft electronics bay) to a hostile environment (e.g., flight controls, engine controls) encode data by modulating the light beam with an electrical signal, and returning the modulated light back to the friendly environment for translation and use. A number of such electro-optic modulators are in use, such as those described by R.A. Becker in his paper, "Broad-Band Guided-Wave Electro-optic Modulators", IEEE Journal of Quantum Electronics, Vol. QE-20, No. 7, July 1984.
A particularly effective electro-optic modulator is described in my U.S. Pat. No. 4,950,000. That electro-optic modulator system basically comprises a device and method for modulating light intensity in accordance with a varying electrical signal and which can receive constant amplitude light through an optical fiber, modulate the light and return the modulated light to the source through the same fiber. The device includes several components positioned within an elongated cavity in a supporting body. a lens at one end of the cavity is arranged to receive light from and end of an optical fiber held against the lens. The lens collimates the incoming light into a narrow collimated light beam. The beam is then polarized and partially retarded to circular polarization and passed through a modulating material to a mirror, which reflects the modulated light back through the cavity and back into the optical fiber.
The modulator materials is capable of rotating beam polarization in response to voltages between two electrodes which are on opposite sides of a thin rib of the modulator material running the entire length of the modulator thus reducing the voltage in proportion to its length and width. Since light passes through the modulator twice, the optical effect of the modulator will be double that in prior art single-pass modulators, effectively further doubling system efficiency. The required voltage is halved and the second or output set of polarizer and lens is eliminated. The returning beam is attenuated by the polarizer as a function of the beam rotation in the modulator, so that the light level returning into and through the fiber is a function of the voltage imposed on the electrodes.
The device described in U.S. Pat. No. 4,950,884 provides outstanding performance. However, changes in strain and temperature adversely affect performance of that electro-optic system, as with other somewhat similar systems.
Thus, there is a continuing need for improved devices and modulator arrangements that eliminate the undesirable effects of temperature and strain changes while leaving the voltage fields unaffected.